I purchased two ribbed tanks (from the dollar bins at Target $2.50 each) and used one to cut out a loose wave pattern, each one a bit longer and wider than the last. This can be done free hand. Double your fabric because you will need two of each length. Leave one side straight, cutting along the ribs as a guide:
Beginning from the center, butt the matching longest ruffles against each other (straight sides together), and pin. Zigzag stitch the two together with one seam. (Warning: there will be much swearing as you inevitably sew the front to the back at some point during this process. Just sayin'.)
I liked mine butted right up against the neck seam at the top.
Now, you can lift each side of the center ruffle and pin and stitch the remaining two ruffles into place, each under the last, on each side:
When the shirt is upright, the wavy fabric pieces fall into a mass of ruffles:
I use it as a bathing suit cover up. It's so cute!
Any fabric that doesn't fray would work for this project, just think of covering the entire front of a tee with fluttery ruffles! Have fun!